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Study Notes: Understanding Check Samples

Often when doing an analysis, a 'check' sample will be tested at the same time and in the same way as the test sample. If the result for the check sample falls within its normal range, the technician will have confidence that the result for the test sample is correct.

The 'normal range' refers to the natural variation that occurs when the analysis is repeated a number of times - you never get exactly the same result every time. Think of the variation an archer, even a good archer, would get when aiming for the bullseye on a target. Returning to the check sample, the normal range can be measured and is called the 'range of acceptance'.

For more information on range refer to Study Notes: Range of Acceptance which can be found in the Resources and Training Room. This study note describes the determination and application of the range of acceptance of the Low Check Sample used for glucose analysis.

A number of check samples can be used with different concentrations of analyte (chemical compound being analysed). The concentrations are chosen to cover the range likely to be encountered during analysis of the test samples.

Check samples used often include:

Blank A sample containing everything except the analyte. The blank will show the background reading or 'noise' of an instrument. Blanks are often used if the sample or sample reagent has an inherent colour that needs to be subtracted from subsequent sample readings.
Check sample Sample(s) of known concentration, used to assess the accuracy of an analytical run. Check samples covering varying concentrations (low, medium & high) can be used when analysing a range of analyte concentrations in the test samples.
Precision check A sample containing the analyte at a known concentration (could use a check sample). The same precision check sample is run a number of times (replicates) and the range of values obtained gives an indication of the precision, or repeatability, of the run.

For more information on accuracy and precision refer to Study Notes: Variability, Accuracy and Precision.

If check sample values fall within their respective acceptance ranges it can be concluded that the analysis went satisfactorily in all respects. Knowing this, the results for the test samples can be relied upon as being correct ie accurate.

On the other hand, if any of the check sample values are not within their respective ranges, the analysis is considered unsatisfactory. This may have occurred as a result of the:

  • operation of instrumentation
  • the preparation, storage and use of reagents
  • preparation of the samples
  • technique of the analyst.

In these circumstances the test sample results are suspect and would not be reported. The run would usually be repeated once the problem(s) had been identified and corrected.

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